Valve selection for strong acid and strong base and other corrosive media

In the case of corros media, corrosion resistance is the most critical aspect of chemical equipment. If the metal materials of chemical valves are not correctly selected, even a slight mistake can damage the equipment or, in cases, cause accidents or even disasters. According to relevant statistics, about 60% of the damage to chemical equipment is due to corrosion. Therefore, when selecting chemical equipment, is necessary to pay attention to the scientificity of the selection of materials. There is a common misconception that stainless steel is a "universal material" and that no matter what the or environmental conditions are, stainless steel is always recommended. This is incorrect and very dangerous. Below, we discuss the key points of material selection for some commonly used chemical media:

1. Sulfuric acid medium valve

Sulfuric acid is one of the most widely used and important industrial raw materials as a strong corrosive medium. The of materials by sulfuric acid varies greatly with different concentrations and temperatures. For concentrated sulfuric acid with a concentration of more than 80% and a temperature below 80, carbon steel valves and cast iron valves have good corrosion resistance, but they are not suitable for high-speed flowing sulfuric acid and cannot be used as materials for pump valves Ordinary stainless steel valves such as 304 (0Cr18Ni9) and 316 (0Cr18Ni12Mo2Ti) are limited in their use for sulfuric acid media. Therefore, pump valves for transporting sulfuric acid usually adopt high-silicon cast iron (with large casting and processing difficulties) or-alloy stainless steel valves (20-series alloys). Fluoroplastics have good corrosion resistance to sulfuric acid, and the use of lined fluorine (F46) is a more economical choice. If the pressure is too high and the temperature rises, the use points of plastic valves will be affected, and only ceramic ball, which are much more expensive, can be chosen.

2. Hydrochloric acid medium valve

Most metal materials are not resistant to hydrochloric acid corrosion ( various stainless steel materials), and high-silicon molybdenum iron can only be used for hydrochloric acid with a concentration of less than 30% a temperature of 50℃. Contrary to metal materials, most non-metal materials have good corrosion resistance to hydrochloric acid, so lined rubber valves and plastic valvessuch as polypropylene, fluoroplastics, etc.) are the best choice for transporting hydrochloric acid. However, if the temperature of such a medium exceeds 50℃ or the pressure exceeds 16 kg, any plastic (including polypropylene, fluoroplastics, and even polytetrafluoroethylene) will not able to handle it. At present, there is no ideal valve on the market, but you can try the newly emerging ceramic ball valve. The advantage of this Voto valve is self-lubricating property, low torque, no aging, and a much longer life than ordinary valves. Its disadvantage is that it is much more expensive than plastic valves.

3. Nitric acid medium valve

Most metals are rapidly corroded and destroyed in nitric acid, and stainless steel valves are the most widely used corrosion-resistant materials nitric acid. They have good corrosion resistance to all concentrations of nitric acid at room temperature. It is worth mentioning that molybdenum-containing stainless steel valves ( as 316, 316L) do not have better corrosion resistance to nitric acid than ordinary stainless steel valves (such as 304, 21), and sometimes they are even worse. For high-temperature nitric acid, titanium and titanium alloy materials are usually used

4. Acetic Acid Medium Valves

    It is one of the most corrosive substances among organic acids.inary steel will be seriously corroded in all concentrations and temperatures of acetic acid. Stainless steel is an excellent material resistant to acetic acid. 316 stainless valves with molybdenum can also be used for high temperature and dilute acetic acid steam. For high temperature and high concentration acetic acid or other harsh requirements such containing other corrosive media, high alloy stainless steel valves or fluorine plastic valves can be selected.

    5. Alkali (sodium hydroxide) medium

    Steel is widely used in sodium hydroxide solution below 80 ℃ and within 30% concentration. Many petrochemical plants still use ordinary at 100 ℃ and below 75%, although corrosion increases, but the economy is good. Ordinary stainless steel valves do not have a significant advantage in resistance to alkaline liquid compared to cast iron. As long as a small amount of iron is allowed to be mixed into the medium, it is not recommended to use stainless steel valves For high temperature alkaline liquid, titanium and titanium alloy valves or high alloy stainless steel valves are mostly used.

    6. Ammonia (ammonium hydrox) medium valves

    Most metals and non-metals have very slight corrosion in liquid ammonia and ammonia water (ammonium hydroxide), only copper copper alloys should not be used.

    7. Chlorine (liquid chlorine) medium valves

    The corrosion resistance of most metal valves to chlorine very limited, especially in the case of chlorine with water. This includes various alloy valves. In this case, PTFE valves are a very good choice. However, chloralkali chemical plants will find that: PTFE valves will increase the torque force and the problem of PTFE aging will be highlighted after a slightly longer time. In this, the leakage that occurs is fatal. You can consider replacing the original ordinary PTFE lined valve with a PTFE lined ceramic ball core, which will have a perfect effect the self-lubricating property of the ceramic and the corrosion resistance of PTFE.

8. Valve for saline water (seawater) medium

Ordinary steel has a low corrosion rate in sodium chloride solution seawater, and generally requires coating protection; various types of stainless steel valves also have a very low uniform corrosion rate, but may cause localized corrosion due to chloride ions, 316 stainless steel valves are usually better.

9. Valve for alcohol, ketone, ester, ether medium

Common alcohol mediums include methanol ethanol, ethylene glycol, propanol, etc., ketone mediums include acetone, butanone, etc., ester mediums include various methylters, ethyl esters, etc., ether mediums include meth ether, eth ether, butyl ether, etc., they basically have no corrosion, and common materials can used. When making specific selections, the properties of the medium and related requirements should also be made reasonably. In addition, it should be noted that ketones, esters, ethers have dissolving properties for many rubbers, and mistakes should be avoided when choosing sealing materials.

There are many other media that cannot be introduced one by one here In short, it is not appropriate to be casual and blind when making material selections. It is necessary to consult relevant information or draw on mature experience more